This invention relates to forming hollow, molecularly oriented articles such as containers of polyalkylene terephthalate such as polyethylene terephthalate (PET), and more particularly to improvements in a preform process for forming articles of such material having certain non-round cross sections wherein material distribution is improved in the vicinity of the ends of the sides.
Containers of molecularly oriented PET are known and desirable for packaging because of superior clarity, impact strength and barrier properties. Prior to this invention, configurations of such containers have been largely limited to circular yet it is frequently desirable for aesthetics and functional reasons such as space conservation to be able to blow mold non-circular containers of oval and triangular shape with good material thickness uniformity in the peripheral direction.
Attempts in the past to form non-round configurations from commodity thermoplastics such as polyethylene and polypropylene in a process wherein prior-shaped preforms are conditioned to molecular orientation temperature before distension in the blow mold entailed establishing special conditions to promote wall thickness uniformity between areas furthest from the lengthwise axis and areas close to the axis. As disclosed in U.S. Pat. No. 3,950,459, such systems usually involved establishing temperature differences in the circumferential direction of the preforms such that portions to expand less were cooler and therefore would stretch less than hotter portions which were to stretch more in the blow mold. If such temperature difference is not well established excessive thinning or even blow out at the extremities of the cross section could be expected to occur. Needless to say, consistent accurate provision of such a differential does not facilitate simplicity of the preform heat treating process.